The math has changed.
It used to be, you paid money to run an ad. A little piece of media, bought and paid for. The audience came with the slot.
Today, of course, the ad is free to run. Post your post, upload your video. Free.
What to measure, then?
Well, one thing to measure is attention. How many likes or shares or views did it get?
But if you're going to optimize for attention, not trust or results or contribution, then you're on a very dangerous road.
It's pretty easy to get attention by running down the street naked (until everyone else does it). But that's not going to accomplish your goals.
When Oreo gets attention for a tweet or Halotop for a horrible ad, they're pulling a stunt, not contributing to their mission.
Yes, the alternative is more difficult. It doesn't come with a quick hit or big numbers. But it understands what it's for. An effective ad is far more valuable than a much-noticed one.